Briefly stated, a brushless DC motor is a motor in which the position of the magnetic poles of a rotor are detected by means of a detector directly coupled to the shaft of the rotor. In response to the detected position, semiconductor switching elements such as transistors, thyristors, or the like are activated so as to continuously generate torque in the motor. Either field windings of a multi-segment permanent magnet is used for the rotor.
The torque is created by application of currents to the stator or field windings in sequential order. Subsequently, each winding current radiates a torque-inducing magnetic flux that moves the rotor. The DC currents are alternately switched about the field to create various current paths that produce magnetic flux orientations in a synchronized fashion. The synchronous magnetic flux results in a torque on the rotor that causes the desired rotational movement. In order to ensure that current is applied to the proper motor phase, sensing devices are used to provide information concerning the position of the rotor. Typically, position information is determined by employing Hall sensors, optical sensors, or resolvers. These sensor systems determine the relative position of the rotor within one electrical cycle. They cannot provide an absolute position. However, the relative position is accurate enough to enable the motor to be started in the correct direction and accelerated to a nominal speed.
Of the various sensor systems available, the best known and most commonly used, especially in motors where economy and small size are of significant importance, are Hall sensors. However, in use, the position of the Hall elements must be very precisely fixed. Further, the heat sensitizing temperature of a Hall element is limited causing deterioration of the motor characteristics under heavy loading. Additionally, Hall devices are notorious for having short life expectancies. Consequently, using Hall devices significantly limits the reliability of a motor. Also, incorporating Hall sensors into a motor inherently increases the size, manufacturing cost, complexity, and power consumption of the motor.
A number of solutions have been developed in an attempt to avoid using sensors. For example, methods disclosed to date include direct or indirect back electromagnetic force or EMF detection as disclosed in V. D. Hair, "Direct Detection of Back EMF in Permanent Magnet Step Motors", Incremental Motion Control Systems and Devices, Symposium, Urban-Champaign, 1983, pp. 219-21 and K. M. King, "Stepping Motor Control", U.S. Pat. No. 4,136,308, January 1979. Other applicable disclosures include a current analysis in B. C. Kuo, A. Cassat, "On Current Detection in Variable Reluctance Step Motors", Incremental Motion Control Systems and Devices, 6th Annual Symposium, Urban-Champaign, 1977, pp. 205-220 and two third harmonic analyses disclosed in P. Ferrario, A. Vagati, F. Villata, "PM Brushless Motor: Self Commutating Prerogatives with Magnetically Anisotropic Rotor", Instituto di Elettriche, Politecnico di Torino, Italia, and R. Osseni, "Modelisation et Auto-commutation des Moteurs Synchrones", EPFL No. 767, 1989. A rotor position location system using short current pulses has been disclosed in "Detection of Rotor Position in Stepping and Switched Motors by Monitoring of Current Waveforms" by P. P. Acarnley et al., printed in Transactions on Industrial Electronics, August 1985.
However, these methods have two major disadvantages: first, they do not provide any information concerning the position of the rotor at standstill; and second, the back EMF, in most instances, is undetectable at low to medium motor speeds. Consequently, the back EMF methods are not a feasible solution to providing motor control at start-up or initial acceleration.
Various methods and apparatuses to determine the position of a rotor at both standstill and slow speed have been disclosed in "Position Detection for a Brushless DC Motor", U.S. Pat. No. 5,001,405; "Closed-Loop Control of a Brushless DC Motor from Standstill to Medium Speed", U.S. Pat. No. 5,117,165; and "Position Detection for a Brushless DC Motor Without Hall Effect Devices Using a Time Differential Method", U.S. Pat. No. 4,028,852. In aggregate, the above-listed inventions determine the rotor's position at standstill and at low speed while the motor is accelerating to a medium speed. At medium speed, a large back EMF is available to enable the traditional back EMF control circuits to function adequately. The motor is then accelerated under the back EMF control system until a nominal speed is attained.
One known effort to control the motor speed at its nominal speed with a high accuracy is disclosed in U.S. Pat. No. 4,876,491. The method described is only applicable when the motor is used in a hard disc drive. In the main, the motor speed is regulated by a number of concentric tracks of information which are written onto the disc drive media. The speed controller reads this stored information and determines the time between two consecutive tracks of information. This measured time is compared to a reference time associated with the desired nominal speed. Depending on the difference between the reference and the measured time, the controller will accelerate or decelerate the motor in an attempt to match the measured time with the reference. This approach has two major disadvantages: first, the motor control system must use an outside source of information, i.e. the stored information on the disc drive; and second, the time between two consecutive track information measurements may fluctuate depending on motor dissymmetry.